1. Field of the Invention
The present invention relates generally to a cold-cathode fluorescent lamp usable as, e.g., a back-light unit for a liquid crystal displaying device. More particularly, the present invention relates to a cold-cathode fluorescent lamp of the foregoing type which assures that a rate of an effective illuminating length of the cold-cathode fluorescent lamp as measured in the axial direction of the latter to the whole length of the same can be improved, and moreover, a yielding rate of the cold-cathode fluorescent lamp produced via a number of production steps can be improved.
2. Background Art
To facilitate understanding of the present invention, a typical conventional cold-cathode fluorescent lamp will be described below with reference to FIG. 6 and FIG. 7. Specifically, FIG. 6 and FIG. 7 show by way of example the structure of a conventional cold-cathode fluorescent lamp 90. The cold-cathode fluorescent lamp 90 includes a tubular glass bulb 91 of which the inner wall surface is coated with a fluorescent material 92 and of which the opposite ends are airtightly sealed with beads 93. Electricity feeding wires 94 extend through the beads 93 so as to enable an opposing pair of electrodes 95 to be held in the tubular glass bulb 91 in the spaced relationship.
Here, each of the electrodes 95 will be described in more detail with reference to FIG. 7 which is a fragmentary enlarged sectional view. As shown in FIG. 7, each electrode 95 is composed of two rectangular plate-like electrode members 95a each of which is preliminarily coated with mercury and getter while maintaining a necessary area. The two plate-like electrode members 95a are affixed to the innermost ends of the electricity feeding wires 94 on the discharging chamber side by employing a spot welding process while they are slantwise parted away from each other in the radial direction.
The innermost end part of each electricity feeding wire 94 is exposed to the discharging chamber side to bear part or all of a discharging load in the tubular glass bulb 91. To assure that a malfunction of spattering does not occur when the fluorescent lamp is turned on, the foremost end part of each electricity feeding wire 94 is composed of a nickel wire 94a having excellent stability, and the remaining part of each electricity feeding wire 94 extending through the bead 93 is composed of a Dumet wire 94b having an expansion coefficient approximate to that of glass. A Dumet wire is a wire formed of Fe-Ni alloys containing approximately 42% Ni, the surface of which is covered with copper. The nickel wire 94a and the Dumet wire 94b are connected to each other at a certain adequate position on the electricity feeding wire 94 by employing a welding process.
When the conventional cold-cathode fluorescent lamp 90 constructed in the above-described manner is used as a back-lighting unit for a liquid crystal displaying device, it is required that the tubular glass bulb 91 be dimensioned to have a very small size represented by an outer diameter of, e.g., about 3 mm and an inner diameter of, e.g., about 2 mm. In view of the foregoing fact, to assure that each plate-like electrode member 95a has a necessary area, it is naturally unavoidable that a length L3 of each electrode 95 as measured in the axial direction of the tubular glass bulb 91 is elongated.
At this time, discharging is effected within the range defined by a shortest distance between both the electrodes 95 located opposite to each other in the tubular glass bulb 91. Thus, the longer the length L3 of each electrode 95, the larger the non-illuminating part of the cold-cathode fluorescent lamp 90. Consequently, a rate of the effective illuminating length of the cold-cathode fluorescent lamp 90 to a total length L4 of the same is reduced.
When the cold-cathode fluorescent lamp 90 is used as a back-lighting unit for a liquid crystal displaying device, illuminating should be effected within the range defined by the foregoing effective illuminating length. Thus, the longer the length L3 of each electrode 95, the longer the total length L4 of the cold-cathode fluorescent lamp 90. Consequently, there arises a problem that the backlighting unit is designed and constructed with large dimensions, causing, e.g., a portable type electronic device apparatus having the foregoing type of liquid crystal displaying device employed therefor to be correspondingly designed and constructed with large dimensions.
In addition, when each plate-like electrode member 95a is spot-welded to the electricity feeding wire 94, there is a tendency that free ends of the plate-like electrode members 95a are expansively parted away from each other. At this time, when the length L3 of each electrode 95, i.e., each plate-like electrode member 95a is long, a quantity of expansive displacement of the plate-shaped electrode members 95a in that way is increased so that the electrode 95 is liable to come in contact with the inner wall surface of the tubular glass bulb 91.
As is well known in the art, after both the beads 93 of the cold-cathode fluorescent lamp 90 are airtightly sealed, it is necessary that mercury vapor is emitted from the electrodes 95 and the getter is activated by heating the electrodes 95 with electric current having a high frequency. However, when the electrodes 95 are heated with electric current having a high frequency while they are brought in contact with the inner wall surface of the tubular glass bulb 91, thermal energy generated from each electrode 95 at about 850.degree. C. is transmitted to the tubular glass bulb 91. Consequently, there arises a problem that the tubular glass bulb 91 cracks, resulting in the yield rate of product of cold-cathode fluorescent lamps being reduced. Further, since each electricity feeding wire becomes complicated in structure, there arises another problem that the product of cold-cathode fluorescent lamps is produced at an increased cost. In the circumstances as mentioned above, many requests have been hitherto raised from users for solving the aforementioned problems.